Low noise amplifiers (LNAs) found in radio frequency (RF) tuners traditionally have a high gain in order to set the noise figure for the rest of the receiver. Unfortunately, the use of these LNAs also creates broadband noise, which appears at the image frequency band defined as (2·fLO−fRF), where fLO represents the local oscillator (LO) frequency and fRF is the RF frequency. The image noise is directly converted into the intermediate frequency (IF) band once it is fed into a mixer. Consequently, the image noise effectively increases the noise figure of the receiver and ultimately reduces the overall sensitivity of the receiver. In a broadband TV tuner based on a dual conversion receiver architecture, the received RF band signals (47 MHz to 870 MHz) are up converted to a higher fixed frequency (e.g., 1220 MHz) and then down converted to a low IF frequency (e.g., 10 MHz). The RF front-end up-conversion stage consists of a low noise amplifier, a variable gain RF attenuator and a RF mixer. For the dual-conversion broadband TV tuner, the image noise induced by a low noise amplifier at front-end may result in more than 3 dB addition to the cascaded noise figure of the receiver. This is potentially a critical issue which is often overlooked and which presents a potentially significant obstacle to achieving a low noise figure of the broadband RF front-end for TV tuner applications. Furthermore, for terrestrial TV applications, interference may exist in the image band. Unattenuated image interference reduces the signal-to-noise-distortion ratio (SNDR) below desired levels. This ultimately degrades the quality of the television signal viewed on the display.
To address these issues, various approaches are generally taken. One approach involves placing a filter immediately after the LNA stage in an effort to suppress any image noise. These filters will typically have a flat response over the frequency band 47 MHz to 870 MHz in order to avoid signal suppression. A conventional design typically involves placing a standalone passive filter before the RF mixer.
A standalone filter requires an extra buffer to drive the passive filter. However, the loss due to the passive filter in addition to the extra buffer stage actually contributes to the total noise figure. Therefore, the level of broadband noise suppression from standalone passive filters is limited. Furthermore, the extra buffer required for a passive filter results in additional power consumption.
Hence, the approach taken to address image noise in an RF front-end suffers from various perceived shortcomings including increased power consumption, additional loss, and increased noise, each of which can potentially lead to increased noise figures for a given system. This ultimately adversely affects receiver sensitivity.